Re: [RFC][PATCH] sched: Fix race in task_group()

Stefan Bader
2012-06-26T17:50:00Z

[-- Attachment #1.1: Type: text/plain, Size: 6286 bytes --]

On 26.06.2012 15:48, Peter Zijlstra wrote:
> Here's one that's actually compile tested (with the right CONFIG_foo
> enabled) and I fixed the autogroup lockdep splat.
>
> ---
> Subject: sched: Fix race in task_group()
> From: Peter Zijlstra <peterz@infradead.org>
> Date: Fri, 22 Jun 2012 13:36:05 +0200
>
> Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
> call task_group() too many times in set_task_rq()"), he found the reason
> to be that the multiple task_group() invocations in set_task_rq()
> returned different values.
>
> Looking at all that I found a lack of serialization and plain wrong
> comments.
>
> The below tries to fix it using an extra pointer which is updated under
> the appropriate scheduler locks. Its not pretty, but I can't really see
> another way given how all the cgroup stuff works.
>
> Reported-by: Stefan Bader <stefan.bader@canonical.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> include/linux/init_task.h | 12 +++++++++++-
> include/linux/sched.h | 5 ++++-
> kernel/sched/core.c | 9 ++++++++-
> kernel/sched/sched.h | 23 ++++++++++-------------
> 4 files changed, 33 insertions(+), 16 deletions(-)
>
> --- a/include/linux/init_task.h
> +++ b/include/linux/init_task.h
> @@ -123,8 +123,17 @@ extern struct group_info init_groups;
>
> extern struct cred init_cred;
>
> +extern struct task_group root_task_group;
> +
> +#ifdef CONFIG_CGROUP_SCHED
> +# define INIT_CGROUP_SCHED(tsk) \
> + .sched_task_group = &root_task_group,
> +#else
> +# define INIT_CGROUP_SCHED(tsk)
> +#endif
> +
> #ifdef CONFIG_PERF_EVENTS
> -# define INIT_PERF_EVENTS(tsk) \
> +# define INIT_PERF_EVENTS(tsk) \
> .perf_event_mutex = \
> __MUTEX_INITIALIZER(tsk.perf_event_mutex), \
> .perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
> @@ -168,6 +177,7 @@ extern struct cred init_cred;
> }, \
> .tasks = LIST_HEAD_INIT(tsk.tasks), \
> INIT_PUSHABLE_TASKS(tsk) \
> + INIT_CGROUP_SCHED(tsk) \
> .ptraced = LIST_HEAD_INIT(tsk.ptraced), \
> .ptrace_entry = LIST_HEAD_INIT(tsk.ptrace_entry), \
> .real_parent = &tsk, \
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1246,6 +1246,9 @@ struct task_struct {
> const struct sched_class *sched_class;
> struct sched_entity se;
> struct sched_rt_entity rt;
> +#ifdef CONFIG_CGROUP_SCHED
> + struct task_group *sched_task_group;
> +#endif
>
> #ifdef CONFIG_NUMA
> unsigned long numa_contrib;
> @@ -2749,7 +2752,7 @@ extern int sched_group_set_rt_period(str
> extern long sched_group_rt_period(struct task_group *tg);
> extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
> #endif
> -#endif
> +#endif /* CONFIG_CGROUP_SCHED */
>
> extern int task_can_switch_user(struct user_struct *up,
> struct task_struct *tsk);
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p,
> * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
> *
> * sched_move_task() holds both and thus holding either pins the cgroup,
> - * see set_task_rq().
> + * see task_group().
> *
> * Furthermore, all task_rq users should acquire both locks, see
> * task_rq_lock().
> @@ -7712,6 +7712,7 @@ void sched_destroy_group(struct task_gro
> */
> void sched_move_task(struct task_struct *tsk)
> {
> + struct task_group *tg;
> int on_rq, running;
> unsigned long flags;
> struct rq *rq;
> @@ -7726,6 +7727,12 @@ void sched_move_task(struct task_struct
> if (unlikely(running))
> tsk->sched_class->put_prev_task(rq, tsk);
>
> + tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
> + lockdep_is_held(&tsk->sighand->siglock)),
> + struct task_group, css);
> + tg = autogroup_task_group(tsk, tg);
> + tsk->sched_task_group = tg;
> +
> #ifdef CONFIG_FAIR_GROUP_SCHED
> if (tsk->sched_class->task_move_group)
> tsk->sched_class->task_move_group(tsk, on_rq);
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -554,22 +554,19 @@ extern int group_balance_cpu(struct sche
> /*
> * Return the group to which this tasks belongs.
> *
> - * We use task_subsys_state_check() and extend the RCU verification with
> - * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
> - * task it moves into the cgroup. Therefore by holding either of those locks,
> - * we pin the task to the current cgroup.
> + * We cannot use task_subsys_state() and friends because the cgroup
> + * subsystem changes that value before the cgroup_subsys::attach() method
> + * is called, therefore we cannot pin it and might observe the wrong value.
> + *
> + * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
> + * core changes this before calling sched_move_task().
> + *
> + * Instead we use a 'copy' which is updated from sched_move_task() while
> + * holding both task_struct::pi_lock and rq::lock.
> */
> static inline struct task_group *task_group(struct task_struct *p)
> {
> - struct task_group *tg;
> - struct cgroup_subsys_state *css;
> -
> - css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
> - lockdep_is_held(&p->pi_lock) ||
> - lockdep_is_held(&task_rq(p)->lock));
> - tg = container_of(css, struct task_group, css);
> -
> - return autogroup_task_group(p, tg);
> + return p->sched_task_group;
> }
>
> /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
>

I ran this version through the testcase and no more warning indeed. Also the
crash is not happening anymore (with the backported version).
This should probably get a "Cc: stable@vger.kernel.org # 2.6.38+" into the s-o-b
area. I don't think 2.6.38..2.6.39 have a longterm support but at least that was
the time when autogroup came in. For 3.0..3.2 the patch needs a bit of tweaking
due to some conference boredom. ;) I am attaching the backport I was using for
the test for convenience. Although ... it has to be refreshed after the original
patch has landed upstream...

Cheers,
-Stefan

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1.2: 0001-sched-Fix-race-in-task_group.patch --]
[-- Type: text/x-diff; name="0001-sched-Fix-race-in-task_group.patch", Size: 5660 bytes --]

From d751ab1f1e532f32412d99b71a1bfea3e5282d07 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Fri, 22 Jun 2012 13:36:00 +0200
Subject: [PATCH] sched: Fix race in task_group()

Stefan reported a crash on a kernel before a3e5d1091c1 ("sched: Don't
call task_group() too many times in set_task_rq()"), he found the reason
to be that the multiple task_group() invocations in set_task_rq()
returned different values.

Looking at all that I found a lack of serialization and plain wrong
comments.

The below tries to fix it using an extra pointer which is updated under
the appropriate scheduler locks. Its not pretty, but I can't really see
another way given how all the cgroup stuff works.

Reported-and-tested-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
[backported to apply to 3.0 and 3.2]
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
---
include/linux/init_task.h | 12 +++++++++++-
include/linux/sched.h | 5 ++++-
kernel/sched.c | 32 ++++++++++++++++++--------------
3 files changed, 33 insertions(+), 16 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 32574ee..13b2684 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -117,8 +117,17 @@ extern struct group_info init_groups;

extern struct cred init_cred;

+extern struct task_group root_task_group;
+
+#ifdef CONFIG_CGROUP_SCHED
+# define INIT_CGROUP_SCHED(tsk) \
+ .sched_task_group = &root_task_group,
+#else
+# define INIT_CGROUP_SCHED(tsk)
+#endif
+
#ifdef CONFIG_PERF_EVENTS
-# define INIT_PERF_EVENTS(tsk) \
+# define INIT_PERF_EVENTS(tsk) \
.perf_event_mutex = \
__MUTEX_INITIALIZER(tsk.perf_event_mutex), \
.perf_event_list = LIST_HEAD_INIT(tsk.perf_event_list),
@@ -155,6 +164,7 @@ extern struct cred init_cred;
}, \
.tasks = LIST_HEAD_INIT(tsk.tasks), \
INIT_PUSHABLE_TASKS(tsk) \
+ INIT_CGROUP_SCHED(tsk) \
.ptraced = LIST_HEAD_INIT(tsk.ptraced), \
.ptrace_entry = LIST_HEAD_INIT(tsk.ptrace_entry), \
.real_parent = &tsk, \
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 56de5c1..1fd9884 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1242,6 +1242,9 @@ struct task_struct {
const struct sched_class *sched_class;
struct sched_entity se;
struct sched_rt_entity rt;
+#ifdef CONFIG_CGROUP_SCHED
+ struct task_struct *sched_task_group;
+#endif

#ifdef CONFIG_PREEMPT_NOTIFIERS
/* list of struct preempt_notifier: */
@@ -2646,7 +2649,7 @@ extern int sched_group_set_rt_period(struct task_group *tg,
extern long sched_group_rt_period(struct task_group *tg);
extern int sched_rt_can_attach(struct task_group *tg, struct task_struct *tsk);
#endif
-#endif
+#endif /* CONFIG_CGROUP_SCHED */

extern int task_can_switch_user(struct user_struct *up,
struct task_struct *tsk);
diff --git a/kernel/sched.c b/kernel/sched.c
index aae0c1d..b99a61e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -746,22 +746,19 @@ static inline int cpu_of(struct rq *rq)
/*
* Return the group to which this tasks belongs.
*
- * We use task_subsys_state_check() and extend the RCU verification with
- * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each
- * task it moves into the cgroup. Therefore by holding either of those locks,
- * we pin the task to the current cgroup.
+ * We cannot use task_subsys_state() and friends because the cgroup
+ * subsystem changes that value before the cgroup_subsys::attach() method
+ * is called, therefore we cannot pin it and might observe the wrong value.
+ *
+ * The same is true for autogroup's p->signal->autogroup->tg, the autogroup
+ * core changes this before calling sched_move_task().
+ *
+ * Instead we use a 'copy' which is updated from sched_move_task() while
+ * holding both task_struct::pi_lock and rq::lock.
*/
static inline struct task_group *task_group(struct task_struct *p)
{
- struct task_group *tg;
- struct cgroup_subsys_state *css;
-
- css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
- lockdep_is_held(&p->pi_lock) ||
- lockdep_is_held(&task_rq(p)->lock));
- tg = container_of(css, struct task_group, css);
-
- return autogroup_task_group(p, tg);
+ return p->sched_task_group;
}

/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
@@ -2373,7 +2370,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
* a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
*
* sched_move_task() holds both and thus holding either pins the cgroup,
- * see set_task_rq().
+ * see task_group().
*
* Furthermore, all task_rq users should acquire both locks, see
* task_rq_lock().
@@ -8765,6 +8762,7 @@ void sched_destroy_group(struct task_group *tg)
*/
void sched_move_task(struct task_struct *tsk)
{
+ struct task_group *tg;
int on_rq, running;
unsigned long flags;
struct rq *rq;
@@ -8779,6 +8777,12 @@ void sched_move_task(struct task_struct *tsk)
if (unlikely(running))
tsk->sched_class->put_prev_task(rq, tsk);

+ tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
+ lockdep_is_held(&tsk->sighand->siglock)),
+ struct task_group, css);
+ tg = autogroup_task_group(tsk, tg);
+ tsk->sched_task_group = tg;
+
#ifdef CONFIG_FAIR_GROUP_SCHED
if (tsk->sched_class->task_move_group)
tsk->sched_class->task_move_group(tsk, on_rq);
--
1.7.9.5

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 900 bytes --]